1. Field of the Invention
The present invention relates to a controller for driving a permanent magnet type synchronous motor (hereinafter referred to as a PM motor).
2. Description of the Related Art
There is a strong demand for the miniaturization of vehicle driving motors for electric vehicles. A PM motor is a type of motor that uses permanent magnets as a generating means for excitation magnetic flux and is characterized by having a large field magnetomotive force per unit volume. Therefore, since the PM motor is easier to miniaturize than other types of motors, various electric vehicles using the PM motor as the driving motor have heretofore been proposed.
Vector control is widely used as a method for controlling the vehicle driving motors of electric vehicles. Vector control is a method where target control is performed by separating the motor current IM into a torque current component Iq and a field current component Id. Of these components, Iq generates a torque (magnet torque) from an interaction with the main flux, namely, the excitation magnetic flux obtained at the permanent magnet. Id generates the excitation magnetic flux that partially strengthens or weakens the main flux. If the motor has pole saliency, Id also generates a torque (reluctance torque) proportional to Id.cndot.Iq.
When using vector control in a system having a battery for a power supply, such as an electric vehicle, the following problems arise. First, within the motor, the main flux E0 interacts with the winding at a rotor angular velocity .omega. of the motor. A voltage induced in the winding by this interaction is called a speed voltage. The speed voltage is an electromotive force that is directly proportional to the rotor angular velocity .omega. of the motor and can be expressed as .omega..cndot.E0. Therefore, as the rotor angular velocity .omega. of the motor rises, the speed voltage .omega..cndot.E0 also rises. As the speed voltage .omega..cndot.E0 rises, the voltage across the ends of the winding rises, and in turn the terminal voltage of the motor rises. If the terminal voltage rises significantly so as to exceed a value corresponding to a battery voltage VB, which is the supply voltage, it can be appreciated that a load is placed on the circuitry and electrical components located between the motor terminals and the battery. To avoid this load, the rotor angular velocity .omega. or the revolution N of the motor must be limited so that the terminal voltage of the motor does not exceed the value corresponding to the battery voltage. This sort of limitation on the operable revolution range is, in other words, an upper limit on the speed range that can be attained in an electric vehicle.
A method called field weakening control has been the most widely used method to solve this sort of problem and is presently handled partially using vector control. Field weakening control is a method for generating the excitation magnetic flux in a direction that weakens main flux E0 through control of Id when rotor angular velocity .omega. of the motor is high, and for further extending the operable region of the motor to the field weakening range at the high revolution side (refer to FIG. 6). Using this method, a high revolution region can be covered even for a motor with a relatively small output. Vector control also includes a mode based on an absolute value and torque angle, which is equivalent to the mode based on Id and Iq, so no distinction is made between them in this application.
Although field weakening control features this sort of advantage, it also results in a drop in efficiency. First, field weakening control increases the absolute value of Id at high revolutions. As mentioned earlier, Id is a current component that contributes little or nothing to torque generation. Thus, if Id (hereinafter referred to as a field weakening current) is excessively large when field weakening control is performed, loss increases. Conversely, if the field weakening current is excessively small, it hinders the achievement of the original object of the field weakening. In other words, the circuitry and electrical components provided between the motor terminals and battery, such as a power converter for motor output control, is subjected to stress so that the required Iq cannot be output. As a method to resolve these problems, the assignee for the present invention has previously proposed a method for varying the value of the field weakening current according to VB (refer to Japanese Patent Laid-Open Publication No. Hei 7-107772). According to this method, the loss generated in field weakening control can be minimized and optimized in the relationship with the voltage or the state of charge of the battery. However, as long as field weakening control is performed, the generation of loss caused by the field weakening current and the resulting drop in system efficiency cannot be eliminated.